1. Technical Field of the Invention
The present invention relates to a semiconductor device having an electrode pad for inspection and a method for producing the same.
2. Description of the Related Art
When inspecting electrical characteristics of a semiconductor device formed on a semiconductor wafer, such a method has been generally employed, by which an inspection is carried out in such a state where an electrode pad for inspection is formed on a device to be inspected, and an inspection probe is brought into contact with the electrode pad. Hereinafter, a description is given of an example (hereinafter called Prior Art 1) for producing a semiconductor device on which an electrode pad for inspection is formed.
FIG. 1A through FIG. 1F are sectional views showing a prior art method for producing a semiconductor device in order of steps. As shown in FIG. 1A, a multilayer film 11 in which a wiring layer and an interlayer insulating film, etc., are laminated, is formed on a silicon substrate 10. A wiring layer, in which a TiN/Ti film 13, A1 wiring 12 and a TiN/Ti film 13 are laminated in order, is formed on the uppermost interlayer insulating film. Next, an interlayer insulating film 14 of a two-layer structure including SiON and SiO2 is formed, and a via-hole 22 which is opened from the surface of the wiring layer is formed.
Thereafter, as shown in FIG. 1B, after an adhered Ti layer (or TiW layer) 15, copper film 17 and TiW film 21 are formed in order on the interlayer insulating film 14, these are patterned to a size suitable for incorporation of a solder ball, and an electrode pad is formed.
Next, as shown in FIG. 1C, after a polyimide film 18 is formed so as to cover the electrode pad, the polyimide film 18 is patterned to provide an opening through which a part of the electrode pad is exposed.
Thereafter, as shown in FIG. 1D, the TiW film 21 on the bottom of the opening is etched off by wet etching process using hydrogen peroxide water (H2O2).
In this state, an inspection of electrical characteristics of a semiconductor device formed on semiconductor wafer is carried out. At this time, as shown in FIG. 1E, a copper oxide film 23 is formed on the surface of the copper film 17.
Subsequently, as shown in FIG. 1F, a solder ball 20 is formed in the opening 19.
On the other hand, Japanese Unexamined Patent Publication No. 20001-174514 discloses a semiconductor integrated circuit that is constructed to provide unevenness on the surface of an electrode pad (hereinafter called Prior Art 2). In the technology described in the prior art 2, a semiconductor integrated circuit corresponding to an inspecting method for which a probe is disposed perpendicular to the wafer surface is disclosed. By forming unevenness consisting of projections and recesses on the surface of an electrode pad and further elevating the wafer stage after the probe is brought into contact with the projection on the surface of the electrode pad, the metal of the projections on the electrode pad surface can be crushed. Resultantly, a metal oxide film existing on the surface of the projection is broken, whereby the clean metal surface existing therebelow is brought into contact with the probe, and a contact point of low resistance can be obtained. Also, it is described that 0.2 μm or so is adequate as the size of the unevenness (height of the projection relative to the recess) crushed by probing. Further, it is described that, since an increase in the size of the unevenness results in an increase of a partially thin area of an electrode pad, there is a limit to the unevenness, which is ⅕ (one-fifth) or so of the film thickness.
In the process of Prior Art 1, when inspecting the characteristics by bringing a probe needle into contact with a bonding pad, there is a problem in that the contact resistance between the probe needle and the pad is uneven. This depends on slipping of the probe needle when probing, because the surface of the copper film 17 is flat. Since the needle slips, the contact of the pad with the needle is not stabilized, and the contact resistance may change whenever measurement is executed, wherein the value of resistance becomes unstable. Also, since it is difficult to check the needle mark after the measurement, it becomes difficult to check a shift of the probe in terms of its position. This is due to a fact that, since the surface of the copper film 17 is flat, it is difficult for scratches by the probe needle to remain thereon.
Further, in the process of the Prior Art 1, as shown in FIG. 1E, since the surface of the copper film 17 in the opening 19 is covered with an oxide film 23, the wettability of solder is worsened when forming the solder ball 20 as shown in FIG. 2F, wherein there are cases where the solder ball 20 cannot be formed in a satisfactory state. For example, voids are generated in the solder ball 20, and in an extreme case, there are cases where solder is not attached onto the copper film 17. Or, there is a problem in that, since an oxide film 23 intervenes between the solder ball 20 and copper film 17, the resistance increases.
Further, according to Prior Art 2, since the Prior Art 2 is intended to stabilize the resistance according to the principle of crushing the projection of a metal on the surface of the electrode pad by means of a probe although it is possible to make an attempt to stabilize the contact resistance, it is necessary to increase the length of the needle entering into the pad by the pressing force in order to obtain a further stabilized resistance value, that is, it is necessary to apply a further larger load thereto. In this case, there is a concern that the elements may be damaged due to application of a load. Recently, although a so-called low-k film has been widely used as an interlayer insulating film of semiconductor elements, the hardness of the low-k film is low, and it is liable to be cracked. Therefore, such a situation arises, in which there is a further concern that the elements may be likely to be damaged due to application of the load.
On the other hand, there are cases where an electrode pad to be inspected is constructed so that, thereafter, a solder ball is formed on the upper part thereof and a bonding pad is formed. With a peripheral electrode structure including the solder ball, a semiconductor element is electrically connected to a peripheral circuit, and at the same time is mechanically supported on a wiring substrate. When forming the solder ball, the layer located on the surface of the electrode pad is required to have satisfactory wettability in regard to solder. Also, the electrode pad is required to have high heat resistivity with respect to solder as a whole.
Further, recently, lead-free composition has been widely utilized for solder balls. The lead-free solder has a very high content ratio in regard to tin. Conventionally, although copper or a copper alloy of low resistance has been used as a metal that composes the electrode pad, tin is able to easily diffuse these metals and becomes a cause of lowering the reliability of elements. Therefore, it becomes necessary that the composition of the electrode pad corresponds to lead-free solder and has such a structure by which diffusion of the above-described tin can be suppressed.
A laminated structure of a barrier metal layer assuming a role of preventing diffusion of tin contained in the solder ball and a contact layer that is brought into contact with solder is favorable as the above-described pad structure. In the case where such a structure is employed, it is impossible that the contact layer is made thick. Therefore, with such a structure, it is difficult to apply the technology according to the Prior Art 2, in which unevenness of 0.2 μm or so is provided, to the electrode pad.